Adenosine is a ubiquitous biochemical messenger. Adenosine binds to and activates certain seven transmembrane-spanning G-protein coupled receptors, eliciting a variety of physiological responses. Adenosine receptors are divided into four known subtypes (i.e. A1, A2a, A2b, and A3). These receptor subtypes mediate different and sometimes opposing effects. In general, activation of the adenosine A2a or A2b receptor leads to an increase in cellular cAMP levels, while activation of the adenosine A1 or A3 receptor leads to a decrease in cellular cAMP levels. A2a adenosine receptors are abundant in the basal ganglia, a region of the brain associated with the pathphysiology of Parkinson's disease. For reviews concerning A2a adenosine receptors, see, e.g., Moreau et al., Brain Research Reviews 31:65-82 (1999) and Svenningsson et al., Progress in Neurobiology 59:355-396 (1999). For a discussion of the role and regulation of adenosine in the central nervous system, see, e.g., Dunwiddie et al., Ann. Rev. Neuroscience 24:31-55 (2001).